Secretary-General Ban Ki-moon said on Friday he had received "grisly reports" that Syrian government forces were arbitrarily executing, imprisoning and torturing people in the battle-scarred city of Homs after rebel fighters had fled.

The Baba Amro district of Homs became a symbol of resistance to Syrian President Bashar Assad after government troops surrounded it with tanks and artillery and shelled it intensively for weeks, killing and wounding civilians cowering in its ruined buildings.

Rebels withdrew on Thursday in a key moment in the year-old uprising against Assad. An official at Syria's Ministry of Foreign Affairs and Expatriates said the army had "cleansed Baba Amro from the foreign-backed armed groups of terrorists".

Opposition activists said Syrian troops were hunting down and killing insurgents who had stayed to cover their comrades' retreat.

"A major assault on Homs took place yesterday," Ban told the 193-nation UN General Assembly in New York. "Civilian losses have clearly been heavy. We continue to receive grisly reports of summary executions, arbitrary detentions and torture."

Syria's UN Ambassador, Bashar Ja'afari, said Ban's remarks included "extremely virulent rhetoric which confines itself to slandering a government based on reports, opinions or hearsay."

"The secretary-general is not duly informed," he said, reiterating that the Syrian opposition consisted of "armed terrorist groups".

The International Committee of the Red Cross said an aid convoy had reached the bombarded Baba Amro area, under siege for 26 days, but was not allowed to enter.

"It is unacceptable that people who have been in need of emergency assistance for weeks have still not received any help," ICRC President Jakob Kellenberger said in a statement.

"We are staying in Homs tonight in the hope of entering Baba Amro in the very near future."

Ban urged Damascus to grant immediate access for aid workers, describing the images of death coming out of the country as atrocious.

One activist in Homs told Reuters: "The Syrian army was holding the convoy up because they want to clean up after what they have done in Baba Amro." As with other activist reports from Homs, this could not be independently confirmed.

"All men who remained in the neighborhood aged between 14 and 50 were arrested. We fear they will be massacred. Where is the world?" said one activist.

"The massacres are continuing. They are torturing them and killing (detainees) one by one. They are executing them in batches," another activist, who left Baba Amro on Friday, told Reuters via Skype.

Many fighters out of the 2,000 who were based in Baba Amro were killed and wounded in the onslaught, they said, adding that a final toll was impossible to give because of the heavy shelling and siege. Hundreds were reported to have fled.

The ICRC convoy of seven trucks carrying food and other aid supplies left Damascus early on Friday for Homs, where it met local volunteers and ambulances of the Syrian Arab Red Crescent prepared to treat and evacuate the sick and wounded.

"It’s a massacre"

Syria said on Friday it wanted to express its "sadness and sorrow" at the death of U.S. journalist Marie Colvin who was killed during the bombardment on Homs last week.

British photographer Paul Conroy, who escaped Homs after suffering leg injuries in the shelling, said there had been a daily, indiscriminate barrage of the city.

"I've worked in many war zones - I've never seen or been in shelling like this," the Sunday Times photographer told Sky News from a hospital bed in central London.

"I'm an ex-artillery gunner so I can kind of follow the patterns - they are systematically moving through neighborhoods with munitions that are used for battlefields.

"It's not a war, it's a massacre, an indiscriminate massacre of men, woman and children."

Defiant protesters took to the streets after Friday prayers in towns and cities across Syria - Homs, Hama, Deir al-Zor, Deraa, Douma and several districts in Damascus, television footage showed.

Activists' video footage appeared to show troops shooting at demonstrators. The activists said 40 people were killed in violence across the country on Friday.

The London-based Syrian Observatory for Human Rights said at least 13 people were killed when troops fired a mortar bomb at a protest in the town of Rastan.

Independent verification of such reports is hard as foreign media are mostly barred from Syria.

"God curse you Hafez for having a son like that," shouted protesters in reference to Assad's father who ruled Syria for three decades until he died in 2000. "The people want to announce Jihad (Holy War)", read a banner.

In Geneva, the United Nations human rights body had earlier reminded Assad of his obligations under international law.

"We are alarmed at reports starting to come out of the Baba Amro district of Homs after it was taken over by government forces yesterday," spokesman Rupert Colville said.

One pro-government figure said troops had "broken the back" of the uprising and the rebel withdrawal heralded impending victory over what he termed a Western-backed insurgency.

A Lebanese official close to Damascus said a defeat for the rebels in Homs would leave the opposition without any major stronghold in Syria, easing the crisis for Assad, who remained confident he could survive.

Heavy snow, food shortages

Conditions in the heavily bombarded district are hellish. TV footage showed heavy snow and freezing weather, with residents lacking electricity or fuel for heating.

There is also a shortage of food and medical supplies.

Barely a building has escaped damage from artillery shelling and many are pock-marked with bullet holes.

In a rare show of unity with Western powers, Russia and China joined other Security Council members at the United Nations in expressing "deep disappointment" at Syria's failure to allow the UN humanitarian aid chief Valerie Amos to visit the country, and urged that she be allowed in immediately.

It was the first statement on Syria from the council, which has been deadlocked on the issue, since August last year.

France said it would shut its Syrian embassy and was ready to step up support of the rebels if the UN Security Council cleared the way for such a move. "Dictators will all, one day, have to pay for their actions," President Nicolas Sarkozy said.

British Prime Minister David Cameron said Syria's rulers would be held to account.

"We need to start collecting the evidence now so that one day, no matter how long it takes, there will be a day of reckoning for this dreadful regime," Cameron told reporters at a summit of EU leaders in Brussels.

The European Union was planning to call for more pressure on Assad, including sanctions, according to a draft of its conclusions. It was preparing to urge the Arab League to convene a meeting of the Syrian National Council, which it said it recognized as a legitimate representative of the Syrian people.

The EU has over the past months been adding names to a list of people it sanctions with travel bans and asset freezes.

The United Nations says Syrian security forces have killed more than 7,500 civilians since the revolt began last March.

Syria's government said in December that "armed terrorists" had killed more than 2,000 soldiers and police during the unrest.

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